1. Field of the Invention
The present invention relates to a scanning display device using a coherent light source and a speckle reduction method for such a scanning display device.
2. Description of the Related Art
Projectors employing laser light sources have recently been developed. Since the laser light source is superior in terms of monochromaticity and longer operating life, expectations are placed on the projectors of this type as next-generation projectors.
However, granular speckle noise attributable to coherence of the laser beam occurs in a projector which uses a laser beam having an aligned direction of polarization, which raises a problem of degradation of image quality.
An image display unit described as a technique for reducing such speckle noise in connection with; for instance, JP-A-2010-217291, has a liquid crystal device between a light source and a display area, wherein it is equipped with a birefringent liquid crystal layer to be activated in a state of dynamic scattering.
Moreover, JP-A-2010-244021, for instance, describes an image display unit that has, in an optical path of a laser beam, polarized light formation means for converting an incident laser beam into axially symmetric polarized light, and a projection plane is scanned with the polarized light to thereby render an image.
However, the speckle reduction technique utilizing scattered light, such as that descried in connection with JP-A-2010-217291, encounters a problem of degradation of image quality, like a contrast, which would otherwise be caused by scattering of a beam when the technique is applied to a scanning display device.
The speckle reduction technique described in connection with JP-A-2010-244021 requires generation of polarized light with an axially symmetric polarized light distribution or an inhomogeneous vector beam. This means that discontinuity inevitably exists in polarization within a beam diameter. Presence of such an area where polarization is discontinuous results in a problem of degradation of beam quality. Specifically, an outgoing beam undergoes diffraction to no small extent in the area where polarization is discontinuous, whereby a beam that is to form a pixel spreads, to thus degrade an image. Even when a vector beam formation element is formed from liquid crystal, an orientation distribution becomes discontinuous at a center of a center axis of a bundle of rays, which induces anomalous orientation. The anomalous orientation accounts for scattering, so that beam quality is degraded likewise.
Moreover, the speckle reduction technique described in connection with JP-A-2010-244021 encounters a problem of impairment of a depolarization function that would arise when an optical axis of the beam is not aligned to an axis of polarization rotation. For instance, suppose the vector beam formation element described in connection with JP-A-2010-244021 is interposed between a scan mirror and a screen. In this case, a beam enters the element while being deflected along a scan direction, and hence the optical axis of the beam cannot be aligned to the axis of polarization rotation at all times. Thus, there is a problem of limitations being imposed on a position where the vector beam formation element is to be placed.
Moreover, when the vector beam formation element cannot be placed in an optical path subsequent to a location where three colors of beams are to be merged because of a short distance of the optical path from the location where the beams are merged to a scan mirror, the vector beam formation element must be placed in each of optical paths of the respective colors of beams before being merged. This raises a problem of an increase in the number of components. Moreover, even when no problems are found in the position where the vector beam formation element is to be placed, there is another problem of requirement of high accuracy in aligning the optical axis of the beam to the axis of polarization rotation in order to yield a high speckle reduction effect.